Mutations in the cardiac RyR gene {RYR2) are associated with Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT), an arrhythmogenic syndrome characterized by the development of adrenergically-mediated ventricular tachycardia in individuals with an apparently normal heart, and have very recently been associated with Hypertrophic Cardiomyopathy (HCM), a major cause of sudden cardiac arrest where excessive cardiac mass leads to abnormalities in contraction, relaxation, conduction and rhythm. Hence, RyR2 mutations may lead to deranged Ca2+ homeostasis, quite possibly the pivotal event for the initiation of tachyarrhythmias (CPVT) and/or pathological structural remodeling (HCM). However, the molecular mechanisms that link a mutation in the RvR2 protein and the development of CPVT or HCM remain incompletelv understood. Our general hypothesis is that RyR2-originated CPVT and HCM phenotypes arise from multiple mechanisms of channel dysfunction, the severity of which is commensurate with the hierarchy of the affected domain in the control of Ca2+ release. We propose that CPVT mutations are normally silent but throw RyR2 channels into catastrophic Ca2+ release under conditions of stress (Padrenergic stimulation), leading to tachyarrhythmias; HCM mutations, on the other hand, elicit chronic and insidious Ca2+ release due to constitutive activation of RyR2 channels, leading to pathological cardiac remodeling. To test these hypotheses, we will use single RyR2 channels, isolated ventricular myocytes and whole hearts from wild-type mice and knock-in mouse models of HCM and CPVT to: (1) determine whether distinct patterns of RyR2 dysfunction emerge from mutations in each of the three CPVT structure-function domains (CPVT-I, CPVT-II, and CPVT-III) and the newly-identified HCM domain; (2) determine whether the presumably diverse RyR2 dysfunctions elicited by HCM and CPVT mutations converge into a single or preponderant cellular mechanism of aberrant electrical activity, a major cause of sudden cardiac arrest; (3) determine if the knock-in mouse models of CPVT and HCM develop similar phenotype and respond equally to p-adrenergic stimulation and other exacerbating factors. We will use an array of state-of-the-art techniques including kinetic analysis of single channel activity by laser photolysis of caged Ca2+, high speed Ca2+ imaging with laser scanning confocal microscopy, recording of aberrant electrical activity in beating hearts, and echocardiography. The proposed experimental design will therefore combine molecular, cellular, whole heart and intact animal studies to elucidate the molecular mechanisms of RyR2-initiated tachyarrhythmias with an unprecedented level of integrative physiology.